You can capture other players' Mines. There are five possible outcomes of any given battle:

1. If the attacker defeats the defender, then the attacker will capture all of the resources that have been gathered by the defender.
2. If the defender defeats the attacker and does not lose any troops, they will continue gathering the same amount of resources as before the attack.
3. If the defender defeats the attacker but loses some of their troops, then the maximum amount of resources that can be gathered will be recalculated based on the losses suffered.
4. If the defender defeats the attacker but loses so many troops that they can't carry all the resources that they've already gathered, the troops will leave the Mine immediately and take with them as much of the given resource as their new maximum carrying capacity allows.
5. If the defender leaves the Mine before the attacker reaches it, a battle will not occur, and the attacking army will start marching back to their respective City instead of capturing the Mine.